update comment

git-svn-id: http://svn.openlayers.org/trunk/openlayers@1505 dc9f47b5-9b13-0410-9fdd-eb0c1a62fdaf

lib/OpenLayers/Map.js

@@ -455,7 +455,13 @@ OpenLayers.Map.prototype = {
   /********************************************************/     
 
     /**
-    * @returns {OpenLayers.Size}
+    * @returns An OpenLayers.Size object that represents the size, in pixels, 
+    *          of the div into which OpenLayers has been loaded. 
+    * 
+    *          Note: A clone() of this locally cached variable is returned, so 
+    *                as not to allow users to modify it.
+    * 
+    * @type OpenLayers.Size
     */
     getSize: function () {
         var size = null;